1. Field of the Invention
The invention relates to a transparent protective layer, in particular to a transparent hard-material layer with a high resistance to scratching and a high thermal stability.
The invention relates specifically to a protective layer for glass-ceramic plates, and to a process and apparatus for coating them, these glass-ceramic plates preferably being used as cooking plates in cooking hobs and having a protective layer with a higher scratch resistance than the uncoated glass-ceramic on at least one side.
2. Description of Related Art
Modern cooking hobs have a glass-ceramic plate as the cooking plate, the glass-ceramic plate typically being planar, although it may also be deformed in two or three dimensions. Glass-ceramic plates are both known from printed literature and commercially available, either in undecorated form or decorated with thermally stable colors, e.g. ceramic colors. The cooking plate has individual cooking zones which are heated inductively, by electrically operated radiant heaters, by gas radiant heating elements or by alternative heating systems (for example DHS produced by SCHOTT).
Glass-ceramic plates typically have a Mohs hardness of from 5–6, which is comparable to that of steel, from which cookware is typically produced. Everyday use, for example the cookware being put down and moved around, and the cooking plates being cleaned with abrasive cleaning agents and sponges or with a scraper, imposes a high mechanical loading on the cooking hob, which can lead to traces of use being produced on the hob.
In addition, the cooking plate is often also used as an additional work surface in the cold state. In particular in this state, there is a high risk of surface damage forming, for example through damage caused by rough bases of ceramic objects. All the surface damage incurred, over the course of time, leads to the formation of scratches on the surface, which are more or less noticeable to the user depending on the selected illumination. An additional factor is that damage to the surface offers points of attack for soiling. The ease of cleaning of the surface becomes restricted, since it is much more difficult to clean dirt out of this damage. This effect is independent of whether the cooking hob is transparent, colored or translucent.
The glass-ceramic plates of previous generations had a typical surface structure which was similar to orange peel. Although these plates were also scratched as a result of the phenomena described above, they had a relatively low susceptibility to scratches on account of the additional surface structure. However, over the course of time, the surfaces of glass-ceramic plates have become smoother and shinier, which for the reasons mentioned above leads to an increased susceptibility to scratching.
EP 0 716 270 B1 describes a cooking plate formed from glass-ceramic, on the top side of which a décor is provided, this cooking plate, in order to avoid scratches and traces of use, having a protective layer in the form of enamel fluxes or a silicate coating with a higher scratch resistance than glass-ceramic, with this protective layer covering the glass-ceramic cooking plate continuously or as continuously as possible, and a décor being printed onto this protective layer or directly onto the glass-ceramic surface. It is preferable for the protective layer to be formed from a dark material. Although this protective layer in principle increases the mechanical load-bearing capacity of the glass-ceramic cooking plates, so that, when the cooking plate is in use the susceptibility to scratches can be reduced compared to an unprotected cooking plate, the enamel fluxes or silicate protective layers which are all that is disclosed by the EP document still do not offer optimum long-term mechanical protection. Specifically, one drawback is that the protective layer itself represents a décor which is applied by means of screen printing. These décor colors are generally based on the same fluxes as the décor colors used for optical design purposes. Therefore, they are subject to the same restrictions in terms of abrasion. The minimum dimension of décors of this type is of the order of magnitude of 0.5 mm, which is in any event visually perceptible and therefore interferes with the design, in particular if glasses or glass-ceramics with smooth surfaces are desired.
Furthermore, the explanations given do not allow any conclusions to be drawn as to the extent to which the proposed solution is compatible with the heater systems used. In particular the use of preferably dark materials as protective layer for glass-ceramics with a high IR transparency and radiant heaters will lead to restrictions in terms of the desired IR transparency and therefore to losses in terms of the initial cooking performance.
DE 100 00 663 A1 describes a process and the associated apparatus for providing an optically transparent body with a scratch-resistant layer of Al2O3 over the entire surface by means of a modified PICVD process, in such a manner that a hard-material layer is formed, since it has been found that the known processes cannot be used to produce a sufficiently hard, dense, scratch-resistant and thermally stable layer, in particular from aluminum oxide. One drawback is the high cost of the process, in particular if large-area coatings have to be applied homogeneously. Hitherto, inhomogeneities have been inevitable, and this moreover has a long-term adverse effect on the visual appearance.
Furthermore, WO 96/31995 describes an inductively heated glass or glass-ceramic cooking plate with integrated coils to which a hard-material layer of Al2O3 is applied by means of the plasma spraying technique, in a layer thickness of between 50 and 200 μm. One drawback in this context is that such thick layers are very rough, and therefore the use properties, such as the abrasion caused by pots and pans, manual abrasion and the cleaning properties are adversely affected. Furthermore, the appearance of the cooking plates having a layer of this type changes completely. The surface appears matt and gray.
Furthermore, it is known from DE 42 01 914 A1 (=U.S. Pat. No. 5,594,231) to provide scanning windows made from glass or glass-ceramic for scanning systems, installed in tills in supermarkets and other retail markets, for recording bar codes applied to the packaging of goods with a transparent hard-material layer on the top side, and then for a transparent coating with sliding properties to be provided on the hard-material layer, to make this scanning window more resistant to wear. Materials which are mentioned as being suitable for the hard-material layer include metal oxides, such Al2O3, ZrO2, SnO2, Y2O3. Aluminum oxide which is deposited in amorphous form is referred to as being particularly suitable. In particular the amorphous deposition of the metal oxide in this context promotes the desired improved hardness and sliding properties of the protective layer. The hard-material layers described here are suitable for applications in the room temperature range, but their properties change at high temperatures, as are customary, for example, in the case of cooking plates, making them unsuitable for use at high temperatures. A protective layer for cooking surfaces requires materials which are able to withstand temperatures of up to 800° C. and which are also able to tolerate the high thermomechanical stresses which occur between the glass-ceramic and the protective layer.
DE 201 06 167 U1 has disclosed a cooking hob with a glass-ceramic plate as cooking plate, this plate being provided with a transparent scratchproof layer which may be formed, inter alia, by a hard-material layer. Metal oxides, such as aluminum oxide, zirconium oxide, yttrium oxide, tin oxide, indium oxide and combinations thereof, are among the materials mentioned for this transparent layer. According to this document, the materials can be deposited, for example, using the sol gel technique, the CVD processes, in particular by means of the PICVD process, and by sputtering.
With the known processes for producing hard-material layers, such as for example those described in the above-mentioned documents DE 42 01 914 A1 and DE 201 06 167 U1, the layers are typically deposited in an amorphous or partially crystalline structure. After prolonged use in the hot areas, or in the event of maximum thermal loading, layers of this type may undergo disadvantageous changes. For example, in these areas the layers may become discolored as a result of thermally induced compacting or may be opacified through crystallization, with the result that the hot areas become optically perceptible. Furthermore, roughening in the range from 1 to 1000 nm may occur. The roughening alone may be optically perceptible, and the recesses which form additionally make cleaning more difficult. The problem of crystallization in the hot areas is exacerbated by mechanical failure of the scratchproof layer. During crystallization, the structure of the layer changes, with the result that cracks are formed in the layer. The loss of lateral cohesion means that the layer no longer offers any particular protection against scratching.
In order, for example, to impart a higher thermal stability to zirconium oxide, it is known (G. Wehl et al., Proc. CVD-VII, 536 (1979)) to add what are known as stabilizers formed from yttrium oxide, magnesium oxide or calcium oxide to this component. However, a layer of this type, produced using the known processes, has a low density, which means that a layer of this type is porous.
The process described in U.S. Pat. No. 4,920,014 for producing a layer of this type from stabilized zirconium oxide attempts to solve this problem by the layer being deposited in such a way, by means of the CVD process and accurately set process parameters, such as temperature of the substrate, instant and duration of the supply of the reaction substances, etc., that it has only one or two crystal planes oriented parallel to the substrate surface. In addition to entailing very high process costs, crystalline layers of this type still have a rough surface.
It is known from the field of turbine technology that layers grown in column form have a particularly high resistance to rapid fluctuating thermal loads. For example, U.S. Pat. No. 4,321,311 describes the use of a ceramic layer which is grown in columnar form as thermal protection for metallic components used in turbine manufacture. However, on account of their coarse crystalline structures, the layers described in this document have a high roughness or porosity.
Rough and porous surfaces quickly become dirty and are difficult to clean. Moreover, they are not visually clear and transparent, but rather are highly diffractive and are unsuitable for applications with visually attractive surfaces.
The scratch resistance problems encountered with other optically transparent bodies formed from glass or glass-ceramic which are exposed to high use temperatures, for example chimney viewing windows, oven windows for pyrolysis ovens, etc., are similar to those encountered with cooking plates.